This invention relates to a novel class of cationic polymerization initiators having a heat-latency, i.e. which are normally inactive but are capable of initiating a cationic polymerization reaction only at an elevated temperature. The invention also relates to heat-curable resin compositions containing these initiators which are useful for the preparation of coating, adhesive, printing ink and other compositions.
A variety of cationic polymerization initiators are known including Friedel-Crafts catalysts such as aluminum chloride, boron trifluoride-ether complex, photo-degradable onium salts (S, Se, Te), diallyl iodonium salts and the like. These known initiators are generally not selective with respect to the reaction temperature. Therefore, an epoxy resin containing these initiators begins to cure even at room temperature.
Japanese Laid Open Patent Application (Kokai) Nos. 37003/83 and 37004/83 disclose another type of cationic polymerization initiators. They are aliphatic or aromatic sulfonium salts capable of generating carbonium cations upon heating to an elevated temperature. Initiators of this type are known as "heat-latent cationic polymerization initiator". Cation-polymerizable resins such as epoxy resins containing the heat-latent initiator are, therefore, normally inactive but capable of curing at a temperature above the cleaving temperature of the initiator. This provides a heat-curable, one-component epoxy resin composion having a greater storage-stability and a longer pot life.
The carbonium cations produced by the thermal cleavage of the heat-latent intiator may react with water or a hydroxy group-containing compound to generate protons which, in turn, catalyse various cross-linking reactions. Accordingly, the heat-latent cationic initiator may find uses in catalyzing the curing reaction of, for example, polyester and acrylic resins with melamine resins. This also provides systems having a greater storage stability.
The heat latent cationic initiator thus has a number of advantages over conventional cationic initiators or proton-donating catalysts. Unfortunately, the prior art sulfonium type initiators have a serious problem in that their sulfur-containing decomposition products are malodorous. This limits their uses in practice.
Accordingly, a strong need exists for a heat latent cationic polymerization initiator which obviates the above defects.